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He said: "We need to treat our waste but it needs to be treated to the best standard available, which is tertiary, and this is just is the bare basics.

"The model of one big plant is flawed in itself. If anything were to go wrong with this plant - and it's happened before, it's happened many times - it would be an environmental catastrophe for the east coast."

"Portmarnock is the only blue flag beach that we actually have in Fingal and they're proposing an outflow pipe to come on to the Portmarnock/Baldoyle estuary with waste treated to the very basic, secondary standard."

Darragh O'Brien (Image: Fianna Fail)

Deputy O'Brien added that he believed alternative plans - such a smaller series of localised plants - should be assessed.

"The last Fine Gael government promised, through Brendan Howlin, to carry out a cost benefit analyses of this and a review of the scheme.

"This scheme is scheduled to cost about €1.2b - that's a lot of money - and there's never been a cost benefit analyses done and no alternatives have been looked at."

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Under Irish Water's proposals, as well as the massive plant in Clonshaugh and the biosolid disposal facility in Dublin 11, there are also plans to build a pump station and odour control unit in Blanchardstown.

There will also be a large sump / inspection chamber constructed behind the beach carpark at Portmarnock, which will see the last remaining sand dunes open to the public closed off for at least a year.

Local businessman Ralph Brady of scuba-diving centre Feel Good Activities in Howth echoed Deputy O'Brien's reservations about the plan.

This map shows where the 2.5m diameter outfall pipe will discharge the waste water (Image: Portmarnock Drainage Awareness)

An 2.5m diameter outfall pipe from the plant will cross Portmarnock Estuary and Baldoyle Bay - a special area of conservation - and will pump secondary treated sewage into the sea just a kilometre the other side of Ireland's Eye.

Seals that call Ireland's Eye home could be under threat if anything were to go wrong at the proposed waste treatment plant (Image: Feel Good Activities, Facebook)

Mr Brady said this would have a hugely negative effect on his business.

He said: "Ireland's Eye would be 90% of our training ground and our dive trips go out there.

"As soon as word is out that they're pumping secondary pollution into the sea, we're zero. No one's going to want to dive."

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Mr Brady added that Ireland's Eye had been revitalised since waste ceased to be pumped in to the surrounding water a number of years ago.

"They've abused Dublin Bay something crazy over the last few years. When they stopped pumping off the head of Howth - which probably would have been about six or eight years ago - the life really came back to Ireland's Eye and the visibility came back."

The closing date for submissions on plans for the Clonshaugh Sewage Treatment Plant is 5.30pm on Thursday, 18 October 2018.